1. Field of the Invention
The present invention relates to a process for producing wet okara (xe2x80x9cokaraxe2x80x9d is insoluble residue from xe2x80x9ctofuxe2x80x9d, or soymilk or soy protein production, and also known as soy pulp). More specifically, it relates to a process for producing wet okara by continuous sterilization.
2. Disclosure or the Prior Art
Okara has been used as a raw material of various food products and, in many cases, dried okara is predominantly used from the viewpoint of preservability. However, dried okara has a problem that it is inferior not only in water retention characteristics and reconstitution properties with water, but also in energy costs compared with those prior to drying.
On the other hand, although wet okara produced by heat sterilization with a batch-wise retort is marketed slightly, there are problems that the wet okara is hardly dispersed due to hardening with time and is liable to be colored, and its taste is spoiled. Further, although wet okara produced by a continuous production process is also known, since it is produced by application of electric current, it is difficult to elevate a temperature of wet okara by application of electric current unless salt is present. Then, when salt concentration is increased, the taste of wet okara is spoiled.
An object of the present invention is to provide a process for producing wet okara by continuous sterilization.
Another object of the present invention is to provide a process for producing wet okara having good preservability without deterioration of water retention characteristics.
These objects and other objects as well as advantages of the present invention will become apparent to those skilled in the art from the following description.
First, the present inventors attempted to use a scraper type heat exchanger as an apparatus for continuous sterilization of conventional okara. However, this continuous sterilization was unsuccessful because the surface of inner wall of the heat exchanger and pumps used were clogged with okara, when conventional okara was used. Then, the present inventors have studied continuous sterilization of wet okara intensively. As a result, it has been found that okara can be sterilized continuously by a scraper type heat exchanger without the above clogging problem by using finely divided wet okara having a specific particle size. Thus, the present invention has been completed.
That is, according to the present invention, there is provided a process for producing wet okara which comprises finely dividing wet okara, feeding it into a scraper type heat exchanger, heating at a temperature of not lower than 120xc2x0 C., cooling the heated product and filling and sealing the cooled product in a container or package aseptically. The wet okara of the present invention has good preservability.
In the present invention, preferably, the finely divided wet okara has a particle size of 10 to 100 xcexcm (the particle size is determined by a coulter counter). Further, preferably, the wet okara has a water content of at least 70% (by weight, hereinafter all percents are by weight unless otherwise stated), and the scraper type heat exchanger has an indirect heating part equipped with at least one scraper blade and outer tubing, and is combined with a forced-feeding part, a holding part and a cooling part.
The okara to be used in the present invention may be that derived from whole soybeans or defatted soybeans. The former okara is mainly obtained from xe2x80x9ctofuxe2x80x9d production and the latter okara is mainly obtained from production of isolated soybean protein using defatted soybeans which has been obtained from soybean oil production.
Suitably, the okara to be used in the present invention has a water content of 70% or more, normally 70 to 95%, preferably 75 to 90%, more preferably 80 to 90%.
When a water content of okara is too low, it is difficult to feed it into a scraper type heat exchanger with a pump at its forced-feeding part. Further, when the okara is eaten after passing through a scraper type heat exchanger, it has gritty mouthfeel, which spoils the taste thereof.
On the other hand, when a water content of okara is too high, okara dispersed in water is precipitated, which causes a watery taste, though a scraper type heat exchanger can be easily driven.
Suitably, the average particle size of okara in a wet state to be fed to a scraper type heat exchanger is 10 to 100 xcexcm, preferably 10 to 50 xcexcm, more preferably 20 to 40 xcexcm. The particle size used herein is that determined by a coulter counter. However, since conventional okara cannot pass through a coulter counter, its average particle size was determined by sieve analysis in a wet state.
When okara has the fine particle size, fluidity is increased and okara can readily pass through a scraper type heat exchanger, thereby facilitating heat sterilization. On the other hand, when okara, such as commercially available one, has a larger particle size, okara is difficult to pass through a scraper type heat exchanger, which causes difficulties in continuous sterilization.
For example, conventional okara obtained from xe2x80x9ctofuxe2x80x9d production has an average particle size of, normally, about 200 to 1,000 xcexcm. When okara having such a larger particle size is try to feed into a scraper type heat exchanger, okara adheres to the inner wall of a pump at its forced-feeding part, and/or the pump is clogged with so-called xe2x80x9cnavel of soybeanxe2x80x9d having a particle size of about 2 to 5 mm, which causes difficulties in constant feeding of okara into the scraper type heat exchanger. Even if a water content is 80% or higher, okara having such a larger particle size shows dry and loose appearance and has no such fluidity as that of the okara to be used in the present invention which has the same water content. Therefore, it is difficult to pressurize okara to feed it into a scraper type heat exchanger. Thus, a commercially available wet okara is sterilized by a batch-wise retort.
The scraper type heat exchanger to be used in the present invention is combined with a forced-feeding part (a) for feeding okara by pressurization.
For feeding okara by pressurization, a feed pump can be used. As the pump, Mono Pump(trademark) can be used generally. However, when okara has considerably high viscosity, a force pump (e.g., screw pump) can be used before Mono Pump(trademark) to stabilize a flow rate.
The scraper type heat exchanger to be used in the present invention has an indirect heating part (b) equipped with at least one scraper blade and outer tubing. The outer tubing is preferably jacketed twofold tubing. Okara is forced to feed into and passing through this outer tubing. Steam (high pressure steam: 2 to 5 kg/cm2, saturated vapor temperature: 120 to 170xc2x0 C.) or water heated to an elevated temperature is passed through the jacket and the inner wall of the jacketed tubing is continuously scraped and renewed with sharp scraper blades (normally, 2 to 4 blades) attached to a rotor part to prevent scorching. Suitably, the scraper blades (the rotor, etc.) are allowed to revolve smoothly on the inner wall of the outer tubing in the indirect heating part to improve heat conduction of okara fed to the scraper type heat exchanger. Further, suitably, the clearance between the inner wall of the outer tubing and the blade (rotor) is 5 to 50 mm, preferably 10 to 30 mm, more preferably 15 to 25 mm to reduce pressure loss of okara fed in the scraper type heat exchanger.
Okara is sterilized by heating at a temperature of not lower than 120xc2x0 C., preferably, 120 to 150xc2x0 C.
The scraper type heat exchanger to be used in the present invention is combined with a holding part (c). Preferably, holding time is 2.6 to 26 minutes in case of sterilization at 120xc2x0 C., 49 seconds to 8.1 minutes in case of sterilization at 125xc2x0 C., 16 seconds to 2.6 minutes in case of sterilization at 130xc2x0 C., 5 to 49 seconds in case of sterilization at 135xc2x0 C., 1.5 to 15.5 seconds in case of sterilization at 140xc2x0 C. and 0.5 to 5 seconds in case of sterilization at 145xc2x0 C. When the holding time is longer than this, the color and taste of okara are spoiled. In addition, okara stored at 5xc2x0 C. for more than 1 week becomes harder and its dispersibility becomes inferior. On the other hand, when the holding time is shorter than this, sterilization is liable to be insufficient, which results in inferior preservability. The holding part may be a tank or tubing.
The scraper type heat exchanger to be used in the present invention is combined a cooling part (d). Suitably, okara is cooled to lower than 100xc2x0 C. in a pressurized state at an outlet of the cooling part. If the heated okara is rapidly cooled to lower than 100xc2x0 C. under normal pressure without providing a cooling part, bumping occurs and air (vapor) escape is caused. This is undesired.
In the present invention, the forced-feeding part (a), the holding part (c) and the cooling part (d) may be combined with one another in one scraper type heat exchanger. Alternatively, they may be separate devices. As the cooling part (d), another scraper type heat exchanger can be used.
Normally, the scraper type heat exchanger is pressurized by a valve at an outlet of the cooling part. In view of workability and sterility, preferably, the pressurization is carried out by two-stage pressurization, i.e., first, pressurizing at the holding part and then pressurizing at the outlet of the cooling part.
When sterilized okara is passed through the holding and cooling parts, a temperature drop of okara is observed due to its high viscosity (even sterilization at 140xc2x0 C., the temperature of okara is dropped to 120xc2x0 C. or lower) and contamination is liable to be caused because of loss of sterility in the holding part and subsequent thereto. Then, suitably, back pressure is applied to the holding part and okara is returned to the original raw material (e.g., a feeding tank) until the temperature of okara at the holding part rises to the desired sterilization temperature. When the okara rises to the desired temperature, it is run into the cooling part. For this purpose, preferably, back pressure-adjusting valves are provided to both holding and cooling parts. In case that the line is relatively long, transportation pressure of the line can be used instead of application of back pressure.
In so far as the above functions are available, any kind of a scraper type heat exchanger can be used in the present invention. For example, a commercially available scraper type heat exchanger, Contherm(trademark) (scraped surface heat exchanger) manufacture by Alfa Laval Contherm Inc. MA, U.S.A. can be used.
Okara thus heat-sterilized is aseptically filled and sealed in a container or package. For aseptically filling and sealing in a container or package, normally, an aseptic filler device for semi-aseptic or aseptic filling can be utilized. In case of semi-aseptic filling, okara can be packed in inner pouches or boxes of a packaging material under a high temperature atmosphere (70 to 90xc2x0 C.). At the same time, this can serve as sterilization of the inner pouches or boxes. Preferably, filling is carried out under an atmosphere of NASA 10000 class or more clean (NASA 10000 is such a state that not more than 0.5 finely divided particle having a particle size of 0.5 xcexcm or larger is present in a space of 1 ft3), thereby preventing contamination of microbes from the outside and adhesion of dust to pillow type packages before using them for packaging due to static electricity.
In case of aseptic filling, after cooling to 10xc2x0 C. or lower by the scraper type heat exchanger, okara can be filled and sealed in pillow type packages (manufactured by Orihiro, Japan) or Scholle(trademark) (bag in box) (manufactured by Toppan Printing Co., Ltd., Japan).
As described hereinabove, according to the present invention, okara can be sterilized continuously.
The following Preparations, Examples, Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof. Hereinafter, all parts are by weight unless otherwise stated.
In Case of High Soaking Temperature Finely divided okara was prepared as follows.
Water (10 parts) were added to dehulled and hypocotyl-removed soybeans (1 part) to carry out soaking at 85 to 95xc2x0 C. for more than 60 minutes. Hot water (90xc2x0 C.) (3 parts) was added to the resultant soybeans sufficiently absorbing water (1 part) (water content: 40 to 50%) and the mixture was treated twice with Comitrol(trademark) (a cutter having rotary blades for cutting a material by shear force; manufactured by Urschel) to obtain a slurry of finely divided soybean particles having an average particle size of 45 xcexcm.
The slurry thus obtained was homogenized by treating twice with a high pressure homogenizer (manufactured by APV) at 200 kg/cm2 to obtain a slurry of finely divided soybean particles having an average particle size of 25 xcexcm.
The homogenized slurry of finely divided particles was centrifuged by treating a centrifuge (manufactured by Tomoe Kogyo, Japan) at 3,000 G for 5 minutes to obtain okara. The resultant finely divided okara had a solids content of 14%.
Commercially available okara from xe2x80x9ctofuxe2x80x9d production contained hulls and hypocotyl of soybeans because it was produced by soaking whole soybeans without removal of hulls and hypocotyl and roughly grinding with a stone mill (e.g., Microcolloider, etc.) . In comparison with that of Preparation 1, this okara had a higher solid content such as 19% and a larger average particle size such as 400 xcexcm or more, and was blackish.
Water (10 parts) was added to dehulled and hypocotyl-removed soybeans (1 part) to carry out soaking at 50 to 65xc2x0 C. for 60 minutes or more. According to the same manner as described in Preparation 1, finely divided okara having a solid content of 14% was obtained.